Well, to Ahab, the white whale signified his Maker - an indifferent, violent, nearly all-powerful nature. By killing the white whale, Ahab was displacing his own Maker and, therefore, becoming a god himself.
So by deposing Whaley, McD destroyed that which made him what he was (hired him) and that which had stood in the way of his complete dominion.
Perhaps that's what he meant?